The disclosed invention relates in general to wafer/mask alignment systems and more particularly to alignment systems utilizing diffraction gratings. Integrated circuits are manufactured by forming on a wafer a set of layers which are patterned to form circuit components and their interconnections. In circuits formed by photolithographic methods, a layer which is to be patterned is first coated with a light sensitive photoresist. The photoresist is exposed by optically imaging a desired circuit pattern on the photoresist layer. The exposed resist is developed to remove either the exposed or unexposed resist depending on the resist type. The pattern is then transferred to the underlying layer by some etching or deposition process such as plasma etching or chemical etching.
An essential requirement in the photolithography process is the accurate alignment or overlay of each successive masking layer to the previous patterned layer. Even in other types of processing, such as direct-write E-beam lithography, it is important to accurately align the wafer with the E-beam device for each step of writing on the wafer. At present, alignment accuracy rather than optical resolution limits the density of alignment system must produce quick, repeatable, high-precision alignment. To improve throughput and yield, it is advantageous
In one type of exposure system known as a step-and-repeat exposure system (stepper), resolution and alignment accuracy are increased by exposing only a small area of the wafer containing one or more integrated circuit images at a time. In general, reduction of exposure field increases resolution. After each exposure, the wafer is moved on an x-y translation stage to the correct location for the next exposure. A typical exposure field size is less than 1 centimeter square, so that typically more than 100 steps are required to expose a 10 cm diameter wafer. In global alignment steppers, each wafer is aligned only once to each mask by using a pair of alignment marks on the wafer to align the x and y positions of the wafer and to rotate the wafer about the z-axis for proper rotational alignment. In field-by-field alignment steppers, alignment is made at each exposure field. Refocussing can also be performed at each exposure field to compensate for wafer distortion introduced for example by processing between masking steps.
An important consideration in the design of an alignment system is the influence of the photoresist process on alignment mark visibility. At present, stepper lenses are designed to produce sharp images at only one or two closely spaced wavelengths. Therefore, the most accurate alignment systems utilize monochromatic light at the exposure wavelength to view the alignment marks on the wafer. Absorption by the photoresist and/or variation in reflectivity due to standing wave effects of the monochromatic light in the resist layer can severely reduce the amount of light reflected from the alignment marks on the wafer, thereby degrading alignment accuracy. The standing wave effects also result in variable exposure of the photoresist, resulting in pattern degradation. To eliminate such standing waves, some photolithography processes utilize below the photoresist a layer which absorbs strongly at the exposure wavelength. In such processes, alignment marks on the wafer are particularly difficult to detect.
There are two general types of alignment processes: in the first, the wafer and the mask are individually aligned to a third object and in the second, the wafer and mask are aligned directly to one another. The GCA 4800 wafer stepper uses a global alignment system in which the mask is manually aligned to the exposure optics by aligning marks on the mask to marks on a mask support platten which itself is aligned to the exposure optics. The wafer is manually aligned to an off-axis alignment microscope (i.e. a microscope which is displaced laterally from the exposure optics) by aligning each of a pair of marks on the wafer to a reference mark in each of the two objectives of the alignment microscope. One of the wafer marks is used for x-y alignment and the other wafer mark is then used for rotational alignment. Following such alignment, the wafer stage is moved a fixed distance controlled by an interferometer to align the exposure field of the wafer with the axis of the exposure optics. The use of separate optics for alignment from that used for exposure, enables each to be optimized for its role, but makes accurate alignment subject to positional stability of the exposure optics, alignment microscope and mask support platten as well as the thermal expansion stability of the interconnecting parts between them.
The GCA sitealigner is an automated field-by-field version of the model 4800 wafer stepper. This stepper utilizes circular Fresnel zone plates as wafer alignment marks. In place of the off-axis alignment microscope is a laser source and a quadrant detector. The zone plate acts as a lens to focus the laser light through associated lenses to produce a bright spot detected by the quadrant detector. The wafer stage is automatically moved in response to signals from the quadrant detector to center the focussed laser light onto the quadrant detector.
In the Censor SRA-100 stepper, four alignment marks on the wafer are illuminated on-axis with light of wavelength to which the photoresist is not sensitive. Because the exposure optics is designed for another wavelength, focal length compensation mirrors are used to adjust the optical path to focus the images of the wafer alignment marks through four windows in the mask to a detector. Fully automatic alignment of the four alignment marks with the four windows occurs at each exposure field.
In a stepper developed by Phillips Research Laboratories, the mask contains a linear grating having lines parallel to the x-axis. A laser beam is directed onto this grating and a pair of holes in an opaque barrier act as a spatial filter to pass only the two first order diffraction beams reflected from this grating. These two orders are imaged onto the mask and interfere with each other to produce a sinusoidally varying intensity on the mask. A birefringent plate splits each of these two orders into two orthogonally polarized beams which are slightly laterally displaced from one another so that two sinusoidal patterns of orthogonally polarized light are produced at the mask. At the point on the mask where these sinusoidal patterns are produced, the mask contains a set of opaque parallel lines having the same spacing as the wavelength of these sinusoidal patterns. Alignment exists when the opaque parallel lines on the mask block an equal amount of each of the two sinusoidal patterns. The light which passes through this pattern of parallel lines on the mask also passes through a polarizer having a direction of polarization that rotates at a constant frequency to alternately pass one polarized sinusoidal pattern and then the other. This light passes on into a detector which produces a signal used to vary the relative position between the wafer and mask to control x-axis alignment. A similar system having grating lines along the y-axis is also present in this system to control y-axis alignment.
In a Cameca alignment system illustrated in FIG. 1, the mask contains a one-dimensional amplitude diffraction grating 11 having grating lines parallel to the y-axis and the wafer contains a two dimensional amplitude or phase grating 12 having rows and columns parallel to the x-axis and y-axis respectively. A laser beam of light is directed through the mask grating and focussed by a projection lens 13 to form on wafer grating 12 an image of the mask grating. If the bright lines of the image of the mask grating lie between the columns of the wafer grating, then the image acts as a one dimensional grating and diffracts light into various diffraction orders in the x-z plane. If the mask grating image lines overlay the wafer marks, then light is diffracted in both the x and y directions. In that system, a first order diffracted beam in the y-z plane is monitored to define alignment in the x-direction. A similar pair of gratings is also used to monitor alignment in the y-direction.
In an alignment system presented by D. C. Flanders et. al. in the article "A new interferometric alignment technique", Applied Physics Letters, Vol. 31, No. 7, Oct. 1, 1977, page 426, the mask and wafer each contain one of a pair of parallel one-dimensional gratings having equal line spacings. A perpendicularly incident laser beam is diffracted into various diffraction orders by the mask grating and each of these diffraction orders that impinge on the wafer grating are further diffracted to produce the output diffraction orders. When the mask grating is aligned directly over the wafer grating, each plus order output diffraction beam is equal in intensity to its corresponding minus order output diffraction beam. In that system, alignment in the x-direction is defined to exist when the +1 and -1 output diffraction orders are equal. A similar pair of gratings are used to detect y-direction alignment. A problem with this system is that blazing of the lines on the mask or the wafer (e.g. by wafer processing steps after production of the wafer grating) can result in misalignment of this system.